1. Field of the Invention
This invention relates to exhaust gas recirculation (EGR) systems on combustion engines, and more particularly relates to the process of mixing the EGR with intake air.
2. Description of the Related Art
Environmental concerns motivate emissions requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. One important group of regulated emission components is the class of nitrogen oxides (NOx) formed during engine combustion.
A system presently in use on many internal combustion engines to retard the formation of NOx is the exhaust gas recirculation (EGR) system. The EGR is mixed with air coming into the engine prior to the air entering the combustion chambers. The blending of EGR and intake air prior to combustion results in lower peak combustion temperatures due to lower concentrations of oxygen in the combustion chamber and the heat-sink effects of inert gas fractions, thus acting to prevent the formation of NOx during combustion. Furthermore, the EGR stream may pass through an EGR cooler prior to mixing with the incoming air, to further lower combustion temperatures and improve the power density of the engine. To ensure the engine runs properly and the emissions are effectively reduced, it is essential to thoroughly mix the EGR with the incoming air such that each cylinder receives an equal gas mixture.
Blending EGR with incoming air introduces competing design constraints. Designs which optimize the mixing of EGR and intake air often introduce significant pressure drop in the system and reduce the performance and efficiency of the engine. Designs which optimize the pressure drop while mixing EGR and intake air often result in poor mixing and inconsistent combustion mixtures reaching each cylinder. Additionally, the available packaging space for installing an EGR system on an engine is often low. Some EGR systems are introduced on engine-vehicle designs that originally did not include EGR, and serious costs are incurred for any extra space consumed by the EGR system. Even where EGR systems are designed into an original vehicle package, space constraints are often significant because increased space usage results in other tradeoffs that increase the cost of the engine and vehicle system.
Further, complicated pipe routing schemes are disfavored because such schemes introduce other constraints into the design of a vehicle system. For example, a pipe carrying EGR gas will typically be hot, and a complex routing scheme for the pipe may limit the places where electronics and other system components can be installed in the engine compartment of a vehicle. Additionally, a complex routing scheme for an EGR system reduces the generality of the engine-EGR design, thereby making an engine-EGR system less able to be dropped into various vehicles without significant redesign costs. Also, complex internal routing schemes with various slots and internal conduits introduce significant machining and manufacturing costs into the system. Further, complex routing schemes reduce transient response times due to large EGR path volumes, reduce transient performance due to inconsistent EGR compositions across the EGR path, and induce pressure drops due to long pipe lengths in the EGR path.
In the current art, several EGR mixing systems are typically used. In a first system, a series of 90-degree straight turns in the EGR system provide some assistance in mixing and help reduce the EGR path volume, but induce significant pressure loss. In a second system, a Venturi is used at the EGR-intake air connection point to reduce the pressure on the intake air side, but these systems provide poor mixing of EGR and intake air. In a third system, a vortex is induced in the intake air where EGR is mixed in. Variations of the third system may introduce added pressure drop through internal conduit flows, and introduce significant manufacturing costs into the system.